The present invention relates to a method of improving the functioning of a membrane of a nanofiltration device, and in particular to improving the functioning of a membrane utilized in a treatment method of a photographic bath.
Many manufacturing or processing methods produce wastewater that, on the one hand, cannot be cleared directly into drains because of their composition and, on the other hand, contain precious substances whose recovery and reuse may have economic advantages. One may cite the example of the photo processing industry, where exposed photographic films and papers go through several treatment baths, after which the chemical products must largely be removed from the finished products. Such treatment methods for photographic films are well known (e.g. see, Chimie et Physique Photographiques; Pierre Glafkides; Vol. 2; Chap. XL, pages 947-967).
It is common to use systems for treating and recycling the wastewater, in particular nanofiltration devices. Nanofiltration is a technique utilized for the selective separation of salts and organic compounds in solution. The membranes used for nanofiltration thus behave like sieves with a large area having microscopic or molecular size pores whose dimensions have to be very even in order for the molecules of a given size to be retained, while smaller molecules or the ions of simple salts pass through the membrane. Membranes for nanofiltration usually let pass molecules whose molecular weight is between 200 and 1,000 daltons. Multivalent ionized salts and non-ionized organic compounds with molar mass more than 1,000 daltons are, however, largely retained. The solution that has crossed the membrane is called the filtrate or permeate and the solution that is retained by the membrane is called the concentrate or retentate.
Nevertheless, it has been noticed that the retention rate of membranes is not optimal during treatment, i.e. a not insignificant amount of substances to be filtered is found in the permeate. This problem is especially important in the nanofiltration systems utilized discontinuously, provided to treat different batches of solution.
An object of the invention is to provide a method to improve the functioning of a membrane of a nanofiltration device provided to filter a solution so as to optimize the retention rate of a membrane.
The invention relates to a method to improve the functioning of a membrane of a nanofiltration device provided to filter a solution, in which the permeate in the solution to be filtered is recycled, and wherein, before the start of utilization of the membrane, the permeate and the retentate are circulated through the nanofiltration membrane for a set time.